About one third of people are susceptible to motion sickness even in mild circumstances such as being on a boat in calm water, although nearly two thirds of people are susceptible in more severe conditions.[2]
Approximately half of the astronauts in the U.S. space program have suffered from space sickness.[3]

Motion sickness on the sea can result from being in the berth of a rolling boat without being able to see the horizon. Sudden jerky movements tend to be worse for provoking motion sickness than slower smooth ones, because they disrupt the fluid balance more.[How to reference and link to summary or text] A "corkscrewing" boat will upset more people than one that is gliding smoothly across the oncoming waves. Cars driving rapidly around winding roads or up and down a series of hills will upset more people than cars that are moving over smooth, straight roads. Looking down into one's lap to consult a map or attempting to read a book while a passenger in a car may also bring on motion sickness.

The most common theory for the cause of motion sickness is that it evolved as a defense mechanism against neurotoxins.[4] The area postrema in the brain is responsible for inducing vomiting when poisons are detected, and for resolving conflicts between vision and balance. When feeling motion but not seeing it (for example, in a ship with no windows), the inner ear transmits to the brain that it senses motion, but the eyes tell the brain that everything is still. The area postrema will always believe the inner ear signal over the eyes, as the eyes are more susceptible to trickery (see optical illusion). As a result, the brain will come to the conclusion that one is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing vomiting, to clear the supposed toxin.

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Airsickness is a sensation which is induced by air travel. It is a specific form of motion sickness, and is considered a normal response in healthy individuals. Airsickness occurs when the central nervous system receives conflicting messages from the body (including the inner ear, eyes and muscles) affecting balance and equilibrium.

Seasickness is a form of motion sickness characterized by a feeling of nausea and, in extreme cases, vertigo experienced after spending time on a craft on water. It is typically brought on by the rocking motion of the craft.

Simulation sickness, or simulator sickness, is a condition where a person exhibits symptoms similar to motion sickness caused by playing computer/simulation/video games.

The most common theory for the cause of simulation sickness is that the illusion of motion created by the virtual world, combined with the absence of motion detected by the inner ear, causes the area postrema in the brain to infer that one is hallucinating and further conclude that the hallucination is due to poison ingestion. The brain responds by inducing nausea and vomiting, to clear the supposed toxin.[4] According to this theory, simulation sickness is just another a form of motion sickness.

The symptoms are often described as quite similar to that of motion sickness. Some can range from headache, drowsiness, nausea, dizziness, vomiting and sweating. A research done at the University of Minnesota had students play Halo for less than an hour, and found that up to 50 percent felt sick afterwards.[5]

In a study conducted by U.S. Army Research Institute for the Behavioral and Social Sciences in a report published May 1995 titled "Technical Report 1027 - Simulator Sickness in Virtual Environments", out of 742 pilot exposures from 11 military flight simulators, "approximately half of the pilots (334) reported post-effects of some kind: 250 (34%) reported that symptoms dissipated in less than 1 hour, 44 (6%) reported that symptoms lasted longer than 4 hours, and 28 (4%) reported that symptoms lasted longer than 6 hours. There were also 4 (1%) reported cases of spontaneously occurring flashbacks".[6][7]

Space sickness was effectively unknown during the earliest spaceflights, as these were undertaken in very cramped conditions; it seems to be aggravated by being able to freely move around, and so is more common in larger spacecraft. Around 60% of all Space Shuttle astronauts currently experience it on their first flight; the first case is now suspected to be Gherman Titov, in August, 1961 onboard Vostok 2, who reported dizziness and nausea. However, the first significant cases were in early Apollo flights; Frank Borman on Apollo 8 and Rusty Schweickart on Apollo 9. Both experienced identifiable and reasonably severe symptoms — in the latter case causing the mission plan to be modified.

One common suggestion is to simply look out of the window of the moving vehicle and to gaze toward the horizon in the direction of travel. This helps to re-orient the inner sense of balance by providing a visual reaffirmation of motion.

In the night, or in a ship without windows, it is helpful to simply close one's eyes, or if possible, take a nap. This resolves the input conflict between the eyes and the inner ear. Napping also helps prevent psychogenic effects (i.e. the effect of sickness being magnified by thinking about it).

Fresh, cool air can also relieve motion sickness slightly, although it is likely this is related to avoiding foul odors which can worsen nausea.[8]

Other treatments for motion sickness rely on medication. Over-the-counter and prescription medications are readily available, such as Dramamine (dimenhydrinate) and Bonine/Antivert (meclizine). Ginger root is a mild anti-emetic and sucking on crystallized ginger or sipping ginger tea can help to relieve the nausea.[How to reference and link to summary or text] Interestingly, many pharmacological treatments which are effective for nausea and vomiting in some medical conditions may not be effective for motion sickness. For example, metoclopramide and prochlorperazine, although widely used for nausea, are ineffective for motion-sickness prevention and treatment. The sedating anti-histamine medications such as promethazine, work quite well for motion sickness, although they can cause significant drowsiness. Scopolamine is sometimes used in the form of transdermal patches (1.5mg) or as a newer tablet form (0.4mg).